OCTOBER 19-20, 2012 - YMCA University of Science & Technology

Proceedings of the National Conference on
Trends and Advances in Mechanical Engineering,
YMCA University of Science & Technology, Faridabad, Haryana, Oct 19-20, 2012
Source : EPRI presentation from www.gasification.org
Technological developments which are expected to be achieved by the next generation of IGCC projects and
contribute to cost reductions include:
• Utilization of dry coal feed system instead of slurry;
• Improvement of gasifier refractory properties, resulting in longer life cycle;
• Ion transport membranes for air separation;
• Gas turbine inlet chilling where appropriate and effective;
• Advanced syngas turbines to increase efficiency and reduce NOx emissions;
• Improved reliability of key components and the overall system in general; and Reduced use of water.
Pulverized Coal (PC) fi ring is the oldest method of thermal power generation. Hot flue gases are used to heat
water in a boiler. The steam produced is used to drive the steam turbine. The waste heat from the turbine is
allowed to condense. It can use any type of coal and is relatively insensitive to the quality of coal burnt. Efforts
have been made to render coal combustion more efficient and less polluting. Fluidized Bed Combustion (FBC)
uses a fluidized bed of fine coal particles suspended in air. At high pressures solid coal behaves like a fluid and
allows rapid transfer of heat. The efficiency of the burning process gets enhanced because the motion of coal
brings a constant supply of hot particles to the surface. The heat is extracted and utilized in a conventional power
generation cycle. It works at lower temperatures than the Pulverized Fuel (PF) process, and hence, reduces NOx
emissions in the atmosphere.
Two operating versions of FBC are Circulating Fluidized Bed Combustion (CFBC) and Pressurized Fluidized
Bed Combustion (PFBC). In Circulating Fluidized Bed Combustion coal particle size is reduced to 0.07–0.3 mm
and the fluidization velocity is kept at 5–10 m/sec, so that the particles are ablated in the steam gas. Since the
gasifier is compact, higher heat release rate per unit area can be achieved. The CFBC can utilize low grade coal
with high ash, or even lignite, and has been adopted in India. Pressurized Fluidized Bed Combustion uses
crushed coal with a limestone suspension as a sorbent (to absorb the sulphur content in the coal). As air pressure
inside the boiler is increased to 16 to 20 bars at a temperature around 850°C, the limestone sorbent captures the
sulphur in the coal and forms a dry paste, which gets collected at the bottom of the boiler and can be removed.
This technique is particularly suitable for high sulphur coals.
Pulverized coal power plants are broken down into three categories; subcritical pulverized coal (SubCPC) plants,
supercritical pulverized coal (SCPC) plants, and ultra-supercritical pulverized coal (USCPC) plants. The primary
difference between the three types of pulverized coal boilers are the operating temperatures and pressures.
Subcritical plants operate below the critical point of water (647.096 K and 22.064 MPa). Supercritical and ultrasupercritical
plants operate above the critical point. As the pressures and temperatures increase, so does the
operating efficiency. Subcritical plants are at about 37%, supercriticals at about 40% and ultra-supercriticals in
the 42-45% range.
Subcritical pulverized coal plant has steam outlet pressure below 22.1 MPa. Typical steam outlet temperatures
(superheat and reheat, respectively) are:538oC/538oC and net plant efficiency (HighHeatingValue(HHV)-basis)
of 35-38 percent for most coals and countries. As an example, a reference plant in the US (subcritical burning
Bituminous coal in standard US ambient and design conditions) is estimated to have plant efficiency of to be
37.7 percent (HHVnet)[6].
• Supercritical pulverized coal plants have steam outlet pressure above 22.1 MPa.Typically, the pressure is 24.7
MPa and the steam outlet temperatures 538- 565oC/565oC resulting in net plant efficiency of 38-40 percent. The
same study estimates that reference supercritical plant in the US would have an efficiency of 39.1 percent
(HHVnet), or 1.4 percentage points higher that the subcritical.
• Ultra-supercritical pulverized coal (USCPC) plants have steam outlet pressure above 22.1 MPa, typically
around 27 MPa, and the steam outlet temperatures in the 565oC to 625oC range. Net plant efficiency in the 42.0-
45 percent range.
• Advanced USCPC plants have steam outlet pressure above 22.1 MPa and steam outlet temperatures above
625-650oC. Typical net plant efficiency: 42.5-46.0 percent.
Capture – ready pulverised coal plant requires :
(a) Design of the steam cycle to better accommodate CO2 compression intercooling.
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